1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a longitudinal magnetic recording medium and a magnetic storage apparatus which are suited for high-density longitudinal magnetic recording.
2. Description of the Related Art
The recording density of longitudinal magnetic recording media, such as magnetic disks, has been increased considerably, due to the reduction of medium noise and the development of magnetoresistive and high-sensitivity spin-valve heads. A typical magnetic recording medium is comprised of a substrate, a seed layer, an underlayer, a magnetic layer where information is written, a C or a Diamond-Like C (DLC) overlayer, and an organic lubricant layer which are successively stacked in this order. For example, the underlayer is made of Cr or a Cr-based alloy, and the magnetic layer is made of a CoCr-based alloy.
The medium noise is reduced by decreasing the exchange coupling between grains, by promoting more Cr segregation in the CoCr-based alloy which forms the magnetic layer. Lowering the medium noise also involves decreasing the grain size and grain size distribution of the magnetic layer, for example, by reducing the thickness of the underlayer. Underlayers which are made of materials such as CrMo, CrTiB, NiAl or the like are presently used in magnetic recording media.
The underlayer described above also promotes crystallographic axis (c-axis or magnetic anisotropy axis) orientation in a plane which increases remanence magnetization of and thermal stability of bits on the magnetic layer. Much success has been realized with an underlayer having a B2 crystal structure, such as NiAl and FeAl which have the (211) texture when deposited on a glass substrate.
However, the (211) texture is weak for the underlayer having the B2 crystal structure, including NiAl and FeAl. For this reason, it is necessary to increase the thickness of the underlayer in order to improve the (211) texture and to obtain a sufficiently high medium coercivity, as compared to a case where a Cr-based alloy is formed on a NiP layer. As a result, there are limitations in controlling the grain size and the grain size distribution of the magnetic layer by way of reducing the thickness of the underlayer, and that it is difficult to further improve the signal-to-noise ratio (SNR) of the magnetic recording medium.
Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium and magnetic storage apparatus, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a magnetic recording medium comprising a substrate, a magnetic layer made of a Co or Co-based alloy, and an underlayer disposed between the substrate and the magnetic layer, where the underlayer is made of an ordered intermetallic material of FCC L12 or FCT L10 crystalline structure. According to the magnetic recording medium of the present invention, the underlayer promotes the formation of small grain sizes and reduced grain size distribution of the magnetic layer.
Still another object of the present invention is to provide a magnetic recording medium comprising a substrate, a magnetic layer made of a CoCrPt-X alloy and having a thickness of 5 to 30 nm, where X=B, Mo, Ta, W and alloys thereof, and an underlayer disposed between the substrate and the magnetic layer, where the underlayer is made of an ordered intermetallic material of FCC L12 or FCT L10 crystalline structure and having a thickness of 3 to 100 nm. According to the magnetic recording medium of the present invention, the underlayer promotes the formation of small grain sizes and reduced grain size distribution of the magnetic layer.
In the magnetic recording medium, the underlayer having the FCC L12 crystalline structure may be selected from a group of Al5CuZr2, Al5CuHf2, (AlCr)3Ti, Al67Cr8Ti25, Al5NiZr2, Al5CuTi2, Al5NiNb2, Al30Dy7Hf3, Al30Dy7Zr3, Al3Er, Al15HfHo4, and Al60Hf7Tb13. Such materials used for the underlayer have the proper crystal structure and lattice parameter to promote epitaxy with the magnetic layer, since Co c-parameter is 0.406 nm.
In the magnetic recording medium, the underlayer having the FCT L10 crystalline structure may be xcex3-TiAl. This material xcex3-TiAl used for the underlayer has the proper crystal structure and lattice parameter to promote epitaxy with the magnetic layer. xcex3-TiAl is FCT, and develops a (001) texture when deposited at a low substrate temperature Ts or at Ts xe2x89xa7200xc2x0 C. This texture results in the c-axis of the magnetic layer being in-plane.
In the magnetic recording medium, the underlayer may be alloyed with at least one element selected from a group of B, Cu, Cr, Hf, Mo, Mn, Ta, Ti, V, Zr or alloys thereof. In this case, it becomes possible to promote smaller grain sizes and to reduce stress in the magnetic layer by the alloying of such elements.
In the magnetic recording medium, the underlayer may be made essentially of tetragonal Al3Ti which is alloyed with an element selected from a group of Ni, Cu, Cr, Mn, Zn, Fe, Co, Ag, Pd, Pt, Au and Rh which make the FCT L10 crystalline structure into FCC L12 crystalline structure. Alloying the Al3Ti with the above elements transforms the L10 crystalline structure, that is, the tetragonal structure, into the L12 crystalline structure, which makes the crystallographic planes such as (100), (010) and (001) have similar dimensions which provide a more uniform lattice for the magnetic layer to grow on.
In the magnetic recording medium, the underlayer may have a lattice parameter a which satisfies 3.9 xc3x85xe2x89xa6axe2x89xa64.3 xc3x85. By setting the lattice parameter a to this range, it provides a good lattice match to that of the magnetic layer, so as to better promote epitaxy.
In the magnetic recording medium, the underlayer may have a multi-layer structure, and each layer of the multi-layer structure may be made of an ordered intermetallic material of FCC L12 or FCT L10 crystalline structure. Some L12 or L10 crystalline structures are better suited for controlling grain sizes and texture, while others may show improvement with thickness but do not grow the proper crystallographic texture when directly grown on a particular surface material. Accordingly, the use of the underlayer having the multi-layer structure can improve the in-plane c-axis orientation even for reduced total underlayer thickness.
A further object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium which includes a substrate, a magnetic layer made of a Co or Co-based alloy, and an underlayer disposed between the substrate and the magnetic layer, where the underlayer is made of an ordered intermetallic material of FCC L12 or FCT L10 crystalline structure. According to the magnetic storage apparatus of the present invention, the underlayer promotes the formation of small grain sizes and reduced grain size distribution of the magnetic layer, thereby enabling improved SNR of the magnetic recording medium.
Another object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium which includes a substrate, a magnetic layer made of a CoCrPt-X alloy and having a thickness of 5 to 30 nm, where X=B, Cu, Mo, Ta, W and alloys thereof, and an underlayer disposed between the substrate and the magnetic layer, where the underlayer is made of an ordered intermetallic material of FCC L12 or FCT L10 crystalline structure and having a thickness of 3 to 100 nm. According to the magnetic storage apparatus of the present invention, the underlayer promotes the formation of small grain sizes and reduced grain size distribution of the magnetic layer, thereby enabling improved SNR of the magnetic recording medium.
Accordingly, the present invention can simultaneously improve the texture of the underlayer and control the grain size and the grain size distribution of the magnetic layer, to thereby enable further improvement of the SNR of the magnetic recording medium.